When a voltage is applied to an organic electroluminescence device (hereinafter, occasionally referred to as “organic EL device”), holes and electrons are injected into an emitting layer respectively from an anode and a cathode. The injected holes and electrons are recombined to generate excitons in the emitting layer. According to the electron spin statistics theory, singlet excitons are generated at a ratio of 25% and triplet excitons are generated at a ratio of 75%.
A fluorescent organic EL device, which uses emission caused by singlet excitons, is applied to a full color display of a mobile phone, a TV set and the like. It has been studied to further improve a performance of the fluorescent organic EL device. For instance, in order to further improve a luminous efficiency, an organic EL device with use of singlet excitons and triplet excitons has been studied.
An organic EL device using delayed fluorescence has been proposed and studied, for instance, a thermally activated delayed fluorescence (TADF) mechanism has been studied. The TADF mechanism uses such a phenomenon that inverse intersystem crossing from triplet excitons to singlet excitons thermally occurs when a material having a small energy gap (ΔST) between singlet energy level and triplet energy level is used. As for thermally activated delayed fluorescence, refer to, for instance. “ADACHI, Chihaya, ed. (Mar. 22, 2012), “Yuki Hando-tai no Debaisu Bussei (Device Physics of Organic Semiconductors)”, Kodansha, pp. 261-262.”
For instance, non-Patent Literatures 1 and 2 also disclose organic EL devices using the TADF mechanism.